Flattened crabs that fold under loose rock, feeding by combing the water with their mouthparts.
Small flattened crustaceans with a smooth rounded carapace to about 15 mm, broad flat chelipeds held out in front, and long slender antennae. They are not true crabs. The last pair of walking legs is reduced and folded up against the body, so only three pairs are visible, and the abdomen is a flap tucked under rather than fused away. Colour is variable and generally cryptic, running through brown, grey, olive and brick red, often marbled or finely spotted.
Feeding is by filtration. The third maxillipeds carry long fringes of setae which are swept through the water in alternating strokes and then wiped clean, and an undisturbed animal on an exposed rock does this continuously. That is the behaviour to look for, because it is more conspicuous than the animal.
They live on the undersides of loose rock and rubble in shallow water, pressed flat against the surface, and they run sideways for cover rather than swimming. A claw shed under attack is not unusual, and porcelain crabs autotomise readily. Identification below the genus needs the ornament of the carapace and the chelipeds and is not reliable from a photograph. Recorded from the shallows to about 20 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
